The present invention relates to processes for dewatering sludges, and more particularly relates to using cellulose in dewatering sludges and products that incorporate the sludges.
Sludges are generated in both municipal water treatment systems and industrial systems, such as papermaking mills.
In the pulp and papermaking industries, the conventional manufacture of paper, cardboard, and related paper products typically results in large quantities of papermaking sludge as a by product. Large paper mills may produce many tons of sludge per day. Pulp and paper sludge can be generated from both influent and effluent water treatment at paper mills, with the majority of sludge generated from effluent water treatment. Influent water treatment relates to clarification to remove suspended matter in large volumes of water that are to be used in the wet-end production process, with river water or other non-treated water being principal sources. Effluent water treatment relates to primary treatment to remove suspended matter in the effluent followed by secondary effluent treatment using biological systems. Typically solids from both the influent and effluent clarification are mixed with waste water sludge produced by secondary biological and chemical-flocculation processes. The combined sludge is conditioned with chemicals, thickened by mechanical methods, dewatered, and disposed of, primarily in a landfill or they are incinerated, or used as a recycled resource.
In more detail, the sludge by product in effluent water arises during papermaking in the following general manner. Paper is conventionally made by draining a low consistency slurry of cellulose fiber pulp, fillers (e.g., clay), and additives (e.g., calcium carbonate or other materials as retention aids, latex dyes, pigments, defoamers, mixing catalysts such as casein or soy protein, etc.) through a paper machine “wire” (essentially an endless mesh or sieve). A certain amount of solid material passes through the wire with the suspension water, and thus it is not retained in the wet paper web formed on the wire. The drained suspension water and suspended solid material is commonly referred to as “white water,” and is reused as far as practicable. However, complete recovery and reuse of this water and papermaking raw materials is not feasible, and a certain proportion is discharged from the paper machine as a waste water stream. This waste water stream from the paper machine typically is treated before it can be discharged from the paper mill. This treatment normally involves passage of the waste water stream through a clarifier or into a settling tank in which heavier components settle to the bottom of the tank and are drawn off as “sludge”. The settled sludge is commonly referred to as the primary sludge. The primary sludge is partly dewatered (e.g., mechanical pressing via a belt filter press, vacuum filter, or screw press), which partly reduces the water content, e.g., to approximately 50% to 80% water content, providing a quantity of sludge for final disposal. Dewatered primary sludges may be dried in a rotary drum dryer, and the like. A secondary source of sludge is comprised of those particles which are too small to settle in any reasonable length of time from the waste water stream during collection of the primary sludge. This overflow can receive a secondary biological treatment and the results of which can be bio-sludge, and the effluent from biotreatment can be chemically flocculated to provide a tertiary sludge. Mixed sludges are formed of combinations of these various sludges, such as primary and secondary sludges.
Papermaking sludges obtained from these sludges often contain large amounts of water, wood fibers, wood particles (“fines”), fillers, additives, together with miscellaneous debris such as grit, sand, plastic particles, dirt, and, particularly if waste paper has been used by the mill as a raw material, ink particles, accumulations of adhesives and other particles used in the paper making process. There is no precise composition for this sludge because there are substantial variations in the wood and other feedstocks used; in the processing materials which must be used to make different types of paper products; and even considerable variation in the processes used by different paper makers in making similar products. This sludge is known in the industry by a variety of names, including: “primary waste treatment sludge”, “paper mill sludge”, “process residual”, “waste treatment sludge”, “waste treatment plant (‘WTP’) sludge”, to name a few. For simplicity, the term “papermaking sludge” is used herein to denote this product, or merely as “sludge.”
Sludge also is generated in the treating of municipal waste waters, and industrial wastewaters besides papermaking. Flocculants often are used in forming such sludges, which are then dewatered by a physical process, such as decanter centrifuge, belt press, or filter press.
Many problems with sludge management are caused by the dewatering characteristics of sludge. In general, the sludge has poor mechanical dewaterability. The dewatering properties will vary depending on the type of sludge, with an acceptable dewaterability for primary sludge and very poor dewaterability for biological and chemical-flocculation sludge. The sludge needs to be dewatered before disposal or used as a recycled resource, or other further handling or use. High water content increases sludge disposal cost considerably and cause other problems.
The present inventors have recognized the need for improved dewatering methods and dewatering additives for sludges, such as industrial and municipal sludges.